The invention relates to a cylinder paper machine, and more particularly to means for controlling paper thickness across the width of the paper produced.
Paperboard is often made on cylinder paper machines, having at least one forming cylinder and often six, seven or more cylinders. A thin ply of paper is formed on each of the cylinders from a dilute suspension of paper fibers and water, and the wet, freshly formed plies are squeezed together before going to a wet pressing section which removes some of the water and more firmly bonds the plies together. The web, which then contains 40 to 60% moisture, goes to a dryer section where the sheet is dried to the desired moisture content.
Each cylinder of the paper machine is formed from a screen which is wrapped around a rigid frame to form a cylinder which is typically 36 to 48 inches in diameter and 50 to 150 inches in length. The screen rotates through a pond holding the suspension of paper fibers and water, with the end of the screen sealed by chime bands which prevent the pond from flowing around the ends of the cylinder. The pond is held by a vat, or in dry vat machines, by a portion of a vat with a seal strip to keep the stock suspension from leaking by the face of the cylindrical screen.
As the cylinder rotates, the pulp suspension is deposited on the wire mesh cylinder. The liquid of the suspension passes through the wire mesh and drains into the cylinder, while the wood pulp fibers remain upon the wire mesh. Near the top of the cylinder there is disposed a couch roll, and the fibers which have now been formed into a thin ply or mat are squeezed or "couched" off the screen to a moving endless blanket called a paper making felt. The ply is pulled away from the cylinder, and the cleaned cylinder portion rotates again into the pond to pick up fresh suspension. The felt moves on until it has picked up plies from all the cylinders operating on the paper machine, and then carries the wet web through the first press section or sections.
The basis weight of the paper board, as measured in grams per square meter, pounds per 1,000 square feet or other units, should be as uniform as possible for any given grade of paperboard. Control of basis weight can be stated in terms of machine direction control (length of the web) or cross machine direction control (width of the web).
Cross machine of basis weight is deter mined by several major factors:
1. Pond consistency: a uniform consistency (fiber to water ratio) of the fiber suspension in the cross direction of the pond is necessary for uniform cross machine pickup of the fibers onto the cylinder. If the consistency is not uniform, the cylinder will pick up more fibers where the fiber to water ratio is higher, and less fiber where the fiber to water ratio is lower. The drainage rate of the stock, as measured by a freeness test, also has an effect on the pick-up rate on the cylinder. However, the consistency across the vat pond and the freeness across the vat pond are closely related, so they will not be separately considered herein;
2. Fiber wash-off: any wash-off of fibers from the mat formed on the cylinder by impingement of inlet stock flow on the mat, either above or below the pond level, will result in uneven basis weight in the cross machine direction, unless the wash-off is uniform across the cylinder;
3. Dirty forming cylinder: any area of the cylinder that is not cleaned will pick up less fiber than the remainder of the cylinder;
4. Excessively high consistency: if the fiber to water ratio of the stock in the pond becomes too high, it will create non-uniform and unstable consistency in the pond. This is a separate problem from the pond consistency discussed above and results from overloading of stock in the pond.
A number of techniques are used by paper makers to adjust the vat pond consistency of the suspension in the cross machine direction, these techniques generally involving the use of a head box, sometimes called a vat inlet section, or weir just upstream of the part. Thus, wood pulp is mixed with water in a stock chest, and the suspension is pumped from the stock chest to the head box. The inlet of the head box is in the bottom portion. The head box normally contains a series of baffles designed to spread the flow uniformly across the width of the machine. The suspension overflows the head box over a making board into the pond.
Wing boards are commonly used to adjust the volumetric distribution of stock suspension flowing into the pond from the weir. A wing board is a barrier placed in the cross machine direction in the upper portion of the head box which includes a central pivot. The position of the wing board is fixed, but the pivot enables the ends of the board to be raised or lowered at the edges of the head box to change the slope of the surface over which the stock suspension flows. The resulting surface is in the shape of a "v", and the volume of the dilute stock suspension in the shallow sides is less than the volume in the deeper center portion. Since there is less dilute stock solution flowing into the edges of the vat pond, this increases the relative consistency at the ends of the pond, and increases the basis weight of the mat formed at those portions of the cylinder.
The use of wing boards is satisfactory to adjust the weight so that it is even on both sides of the machine, but is not satisfactory for finer adjustments across the machine.
The wing board can also be installed with its center raised above the making board, with the board pivoting into a "v"-shape or inverted "v"-shape. In the inverted "v"-shape, the wing board can be used to increase the weight of the center section of machine relative to the edges.
Compound wing boards are also known, compound wing boards including, in addition to a central pivot, pivoting portions at both ends. The pivoting end portions are upwardly movable only. Compound wing boards provide only a limited improvement in adjustment capability over wing boards.
Other techniques for controlling cross machine consistency include dams or restrictors located at the ends of the making board, or "irons" or restrictors placed where needed on the making board, to reduce the amount of inlet flow into the pond at those points, "tins" or extenders on the making board to carry the inlet flow closer to the cylinder, so it impinges on the mat over a particular cross machine section, thus washing some fibers off the mat, and injection of water into the pond or inlet to change the consistency of a specific cross machine section.
Another technique is the use of a sectional metering slots in the inlet flow all the way across the inlet to control the inlet flow by sections. This technique is a described in U.S. Pat. No. 5,792,319, which shows in FIG. 7 corresponding overflow plates at the top portion of the head box. The sectional metering is accomplished by means of a plurality of pivoting sections disposed in the cross machine direction. These pivoting sections can be remotely controlled, and provide good control but are more costly and complex than wing boards.